The present invention relates to a fuel control for an internal combustion engine designed to operate with a lean air/fuel ratio, and more particularly to a control that detects combustion instability due to the use of low volatility fuel during cold starting and warm-up, and for compensating the fuel control to alleviate detected instability.
It is well known that motor vehicle fuels contain a variety of different compounds, and that the fuel volatility can therefore vary significantly from tank to tank. In an effort to quantify this variation, the industry has developed a driveability index (DI) based on a summation of specified distillation measurements, weighted to reflect their relative effect on combustion stability an internal combustion engine. In general, high volatility fuels have a relatively low DI value, whereas low volatility fuels have a relatively high DI value.
In an engine fuel control, the volatility of the injected fuel is most critical during cold starting and warm-up, since the internal surfaces of the engine may not be hot enough to vaporize a sufficient quantity of high DI fuel. For this reason, automotive fuel controls have traditionally been designed to enrich the cold calibration to ensure that the engine will start and run acceptably with high DI fuel. This enrichment to compensate for high DI fuel causes the air/fuel to be richer than optimum with medium and low DI fuel, resulting in higher hydrocarbon emissions than if the appropriate calibration were used. Thus, low emission engines designed to operate at leaner air/fuel ratios during cold starting and warm-up may experience degraded driveability due to combustion instability if a high DI fuel is being used. Accordingly, what is needed is a control method that permits the use of lean air/fuel ratio control during cold starting and warm-up while minimizing degraded driveability due to the use of high DI fuel.
The present invention is directed to an improved engine fuel control that detects combustion instability due to the use of high DI fuel during an engine idle period following starting and that compensates the fuel control for detected combustion instability through temporary enrichment of the delivered air/fuel ratio. When the engine idle speed error magnitude is less than a calibrated threshold, usage of high DI fuel is detected by identifying a surge signal based on the engine speed error fluctuation in a predetermined frequency range attributable to combustion instability due to the presence of high DI fuel in a cold engine. The speed error fluctuation content in the predetermined frequency range is identified with a Butterworth bandpass filter, and the bandpass filter output is low pass filtered to identify an average amplitude of the surge signal. When the engine speed error magnitude exceeds the calibrated threshold, the inputs of bandpass and low pass filters are set to zero. When the average amplitude of the surge signal exceeds a calibrated surge threshold, the presence of high DI fuel is detected. Additionally, the method is disabled for a prescribed period following commanded load transitions associated with the air conditioning system and the automatic transmission. When high DI fuel is detected, the air/fuel ratio of the engine is temporarily enriched to alleviate the instability and to ensure acceptable driveability without unnecessarily increasing hydrocarbon emissions in the engine exhaust gases.